This invention is generally directed to a system for automatically calibrating the output of a D.C. transducer.
Transducers such as photo cells, strain gauges, Hall effect devices and the like generate D.C. output voltages in response to their sensed inputs. Typically, each such transducer has a wide dynamic range (in excess of 1000 to 1) within which it may operate. Because the output signals from such transducers usually have a small magnitude, amplification of the output signals is provided prior to their being further processed. However, the amplifiers and the further processing circuitry for the transducer's output signals generally have a useful dynamic range which is much smaller than the dynamic range of the transducer itself. Hence, the usable range of the transducer becomes limited by the range within which the signal processing circuitry operates. This is one problem which the present invention addresses.
Another problem which conventional transducer calibration systems face is the need for periodic adjustment. For example, a so-called "probe" is used in certain types of retrieval systems in order to optically sense blips or marks on the film as the film is transported. By counting the blips, the film can be positioned for viewing a particular image. Typically, the probe senses a blip by measuring the amount of light which is projected through the film from a light source. As the light source ages or as dust collects on the probe, the probe must be recalibrated to permit accurate sensing of the blips. Analogous problems arise with other types of transducers which also must be recalibrated periodically.
U.S. Pat. No. 2,421,922 describes a technique for automatically recalibrating a probe used in a document reader. In document readers, the documents are transported almost nonstop through the machine, wherefore the probe's sensitivity need not be held at a constant value for a relatively long period of time while the probe's light source is blocked by a document. In contrast, the film in a film retrieval system is frequently parked in one position for a relatively long duration, thus requiring automatic probe calibration of a different type. This difference and the foregoing problems associated with conventional transducer calibration systems illustrate the need for an improved calibration system for D.C. transducers, and particularly for the types of transducers or probes which are used in film retrieval systems.